Field of the Invention
The present invention relates to a liquid developer for use in image-forming apparatuses that utilize an electrophotographic system, for example, electrophotography, electrostatic recording, and electrostatic printing.
Description of the Related Art
An electrophotographic system is a method in which printed material is obtained by uniformly charging the surface of an image bearing member such as a photosensitive member (charging step), forming an electrostatic latent image by photoexposure on the surface of the image bearing member (photoexposure step), developing the thus formed electrostatic latent image with a developer that contains colored resin particles (development step), transferring the developer image to a recording medium such as paper or plastic film (transfer step), and fixing the transferred developer image to the recording medium (fixing step).
The developers here are broadly classified into dry developers and liquid developers: colored resin particles constituted of a material that contains a binder resin and a colorant such as a pigment are used in a dry state in the former, while the colored resin particles are dispersed in an electrically insulating liquid in the latter.
The need for color output and high-speed printing from image-forming apparatuses that use an electrophotographic system, e.g., copiers, facsimile machines, printers, and so forth, has been increasing in recent years. In the realm of color printing, the demand for high-resolution, high-quality images has resulted in demand for developers that can accommodate high-speed printing while having the ability to form high-resolution, high-quality images.
Liquid developers are known to be developers that offer advantages with regard to color image reproducibility. With a liquid developer, the occurrence during storage of aggregation of the colored resin particles in the liquid developer is suppressed, and due to this, a microfine toner particle can be used. As a consequence, excellent properties with regard to the reproducibility of fine line images and the reproducibility of gradations are readily obtained with a liquid developer. Development is becoming quite active with regard to high-image-quality, high-speed digital printing apparatuses that exploit these excellent features by utilizing electrophotographic technologies that use liquid developers. In view of these circumstances, there is demand for the development of liquid developers that have even better properties.
A dispersion of colored resin particles in an electrically insulating liquid, e.g., a hydrocarbon organic solvent or silicone oil, is already known as a liquid developer. However, when the electrically insulating liquid remains present on the recording medium, e.g., paper or plastic film, this ends up causing a substantial decline in the appearance of the image, and due to this the electrically insulating liquid must be removed. In a method generally used to remove the electrically insulating liquid, thermal energy is applied to volatilize and remove the electrically insulating liquid. However, this has not necessarily been preferred from an environmental and/or an energy-savings perspective when the emission of an organic solvent vapor from the apparatus has been a possibility at this point and/or when large amounts of energy have been required.
As a countermeasure to this, a method has been introduced in which the electrically insulating liquid is cured by photopolymerization. A photocurable liquid developer used here uses a reactive functional group-bearing monomer or oligomer as the electrically insulating liquid and has a photoinitiator dissolved therein. This photocurable liquid developer can also accommodate high speeds because it is cured by the reaction of the reactive functional groups upon exposure to light, e.g., ultraviolet light. Such a photocurable liquid developer is proposed in Japanese Patent Application Laid-open No. 2003-57883. Acrylate monomer, e.g., urethane acrylate, is provided in Japanese Patent Application Laid-open No. 2003-57883 as an example of the reactive functional group-bearing monomer.
Japanese Patent No. 3,442,406 proposes the use, as a curable electrically insulating liquid, of a curable liquid vehicle that has a prescribed range for its viscosity and a prescribed range for its resistance value. Epoxy compounds, vinyl ethers, and cyclic vinyl ethers are given as examples of the curable liquid vehicle.